Wheelslip is a derived quantity that indicates the amount of slippage between a wheel and the ground. Wheelslip is important because, by controlling the wheelslip, the force produced by the tires can be directly modified since tire forces are a function of wheelslip.
Controlling wheelslip involves controlling the actual slippage of the wheels and it has the potential to yield precise forms of anti-lock braking, stability and handling enhancement, and traction control. In spite of these potential benefits, practical use of wheelslip control has not become common. There have been three major impediments to the application of wheelslip control. First, wheelslip control requires monitoring the wheelslip level, but calculating wheelslip requires knowing the speed of the vehicle relative to the ground. Second, in its fullest form, wheelslip control requires a brake-by-wire brake system with many actuators and transducers. Third, in its fullest form, wheelslip control depends on knowing the tire/road interface conditions, which change frequently.
Recent advances have produced improved means for solving these three major problems. As a consequence, the commercial application of wheelslip controller brake systems has become more feasible.
For some time, there has existed a mathematical theory for determining the most appropriate wheelslip levels to maintain at the wheels of a ground vehicle during emergency (maximum effort) stops. This theory shows how to find an optimal set of wheelslips that provide the maximum amount of deceleration possible while maintaining the vehicle directional stability within specified bounds. However, the theory does not immediately extend to situations where the driver may be requesting little or no braking or may be accelerating.
There are three possible situations that can arise. In the first situation, the driver is demanding more deceleration than can be provide while maintaining stability. A typical example would be when the driver is braking hard and attempting to turn at the same time. If the brakes are applied at full force, the vehicle might lose control, be difficult to steer, or both. Therefore, it may be desirable to partially release some of the brakes. In the second situation, the driver is demanding a moderate level of deceleration and is only partially applying his brakes. Under such situations, there will be many ways the brakes can provide the demanded deceleration, and some of these ways can also help the vehicle handle better and maintain stability. In the third situation, the driver is demanding less deceleration than is required to maintain stability. A typical example would be where the driver is not applying the brakes at all, and is trying to make a severe maneuver. If the vehicle starts to slide, spin out, or experience excessive understeer, it will be desirable for some brakes to come on to prevent a severe loss of control.
What is needed is the ability to derive individual wheelslip commands for the wheels of a ground vehicle. The derived wheelslips should be chosen to satisfy the driver's demand for acceleration, as much as possible, while at the same time maintaining the directional stability of the vehicle within specified bounds. The wheelslip commands should be generated in a computationally efficient manner.